Combined Sewer Overflow Control Research Articles

Design Issue Analysis and Operation Effect Evaluation of Large-Scale Storage Tank

In order to address the issue of combined sewer overflows (CSOs), W city has constructed a large-scale storage tank with a volume of 220,000 m3. The storage tank is planned for CSO control in the near term and stormwater runoff pollution control in the long term. However, the actual operation of the storage tank is unsatisfactory. This paper elucidates the design scheme and operation mode of the tank and analyzes the challenges encountered during its design and operation. A storm water management model (SWMM) model was constructed to simulate the effect of the storage tank working in a combined sewer system (CSS), a separate sewer system (SSS) and a decentralized storage situation. This study determined that during the 2022 rainy season, the actual reduction in pollutants by the storage tank was only about 60% of the designed value. As a result, the inadequate treatment capacity of the downstream wastewater treatment plant (WWTP) resulted in the water being retained in the tank for a long time, leading to unsatisfactory operation outcomes. If the storage tank works in SSS and the problem of water retention can be solved, it could reduce the total runoff volume by 30% and the total amount of pollutants by 40% during the same rainy season. At the same time, under the premise of constant total storage volume, if decentralized storage tanks were used to control runoff pollution, the reduction effect can be increased by up to 11.6% compared with that of the centralized storage.

Optimization of LID Strategies for Urban CSO Reduction and Cost Efficiency: A Beijing Case Study

Combined sewer overflow (CSO) can lead to serious urban water environment pollution and health risks to residents. Low Impact Development (LID) facilities are one of the important measures to alleviate CSO and have been widely applied. The rational selection of LID facility types, locations, and scales is the most important task, which can effectively improve resource utilization efficiency. Based on the NSGA-II multi-objective optimization algorithm and coupled with the SWMM sewer network hydraulic model, this study takes the combined sewer overflows and the construction cost of LID facilities as optimization objectives and optimizes the types and scales of LID layout in the study area, including eight different return periods. By using the Pareto frontier and visualizing the results of the model, the effects of different rainfall return periods on the CSO control and investment cost of LID layout schemes are compared. The results show the following: (1) the optimization model can demonstrate the relationship between CSO control volume and LID construction cost under different LID layout schemes through the Pareto frontier, showing three different trends, indicating that the relationship between overflow volume and investment cost is nonlinear; (2) with the increase in rainfall intensity, higher requirements are proposed for LID schemes to meet CSO control targets, leading to a decrease in the number of Pareto frontier solution sets. Under larger rainfall intensities, it is difficult to achieve the same overflow control effect by increasing the scale of LID construction. Therefore, considering constraining the LID construction cost between RMB 5.3 and 5.38 million is helpful to determine the most suitable solution; (3) in the optimal layout schemes under different return periods, 87.3% of the locations where LID is deployed have similar scales. Based on these locations with a relatively large proportion of deployment, it can be determined that special attention should be paid to spatial positions in LID planning and construction. This study provides valuable insights for solving combined sewer overflow problems and optimizing urban drainage management and provides guidance for future planning and decision-making processes.

Bibliometric analysis of global performance and trends of research on combined sewer overflows (CSOs) from 1990 to 2022.

Combined sewer overflows (CSOs) are one of the main sources of pollution in urban water systems and significantly impede the restoration of water body functionalities within urban rivers and lakes. To understand the research and frontier trends of CSOs comprehensively and systematically, a visual statistical analysis of the literature related to CSOs in the Web of Science core database from 1990 to 2022 was conducted using the bibliometric method using HistCite Pro and VOSviewer. The results reveal a total of 1,209 pertinent publications related to CSOs from 1990 to 2022, and the quantity of CSOs-related publications indicated an increasing trend. Investigations of the distribution and fate of typical pollutants in CSOs and their ecological effects on receiving waters and studies on pollution control technologies (source reduction, process control, and end-of-pipe treatment) are the current focus of CSOs research. CSOs pollution control technologies based on source reduction and the monitoring and control of emerging contaminants are at the forefront of scientific investigations on CSOs. This study systematically and comprehensively summarized current research topics and future research directions of CSOs, thus providing a reference for CSOs control and water environment management research.

An Evaluation System for Assessing the Operational Efficiency of Urban Combined Sewer Systems Using AHP—Fuzzy Comprehensive Evaluation: A Case Study in Shanghai, China

In recent years, China’s urban smart water management has focused on enhancing the quality, efficiency, energy conservation, flood prevention, and control of combined sewer overflow (CSO) and urban waterlogging. To evaluate the combined sewer system’s operation efficiency effectively, a comprehensive evaluation system was established. This system incorporated expert scoring and the coefficient of variation method, considering 31 specific indexes to assess six key aspects: CSO control, waterlogging control, stable wastewater transportation, pipeline management and maintenance, energy conservation, and smart water affairs. The AHP—Fuzzy comprehensive evaluation was employed for evaluation, combining AHP for index weighting and fuzzy comprehensive evaluation for quantitative scoring. The system was applied to assess the operation efficiency of a specific area in Shanghai from 2020 to 2022. Results showed progress in CSO control and energy conservation, with operation efficiency improving from low in 2020 to moderate in 2021 and good in 2022. However, waterlogging control and pipeline management still require improvement in the combined sewer area. Overall, the evaluation system provides valuable insights into the system’s performance, identifying areas for targeted enhancement and emphasizing the need for further improvements to achieve optimal operation efficiency.

Constructed wetlands for the treatment of combined sewer overflow upstream of centralized wastewater treatment plants

Combined sewer overflows (CSOs) constitute nowadays a major environmental concern, representing one of the main untreated sources of pollution for receiving water bodies. Additional, in recent years, the frequency and intensity of CSOs have increased due to the growing urbanisation and climate change. Although there is no single effective strategy, one of the most promising approaches for the control and treatment of CSOs can be found in nature-based solutions (NBS), such as constructed wetlands (CWs). Despite the demonstrated potential of CWs to treat CSOs, the only data reported in the literature refer to CSO-CW systems located along the sewerage network. This research represents, to the best of our knowledge, the first monitoring of a full-scale CSO-CW designed to treat the combined sewer overflow upstream of the WWTP located in Carimate, in the province of Como (Italy). The system is a multistage CW, composed by a 1st vertical subsurface flow (VF) stage (total area of 8500 m2) and a 2nd free water surface (FWS) stage (total area of 4500 m2). The observed removal rates during the 3 years monitoring campaign (mean values equal to 83.2%, 64.6%, 63.8% for COD, TP, and N-NH4+ respectively) confirmed the treatment efficiency of CW system for all the investigated parameters, consistent with literature data for CSO-CWs in line with the sewerage network, despite its large scale and different hydraulic characteristics. Furthermore, the system also provides additional ecosystem services such as flood mitigation (average CSO interception of 82%), enhancement of biodiversity and a low operational and maintenance cost.

How reliable – and (net) beneficial – is the green in green infrastructure

AbstractThe idea of green infrastructure (GI) has generated great interest and creativity in addressing a range of challenging and expensive environmental problems, from coastal resilience to control of combined sewer overflows (CSOs). The appeal of GI stems from its cost savings compared to traditional “gray” infrastructure and the multiple benefits it provides, including biodiversity, aesthetics, and carbon sequestration. For example, a “green” approach to controlling CSOs in New York City saved $1.5 billion compared to a “gray” approach. Despite these advantages, GI still does not have detailed design and reliability specifications as compared to engineered gray infrastructure, potentially hindering its adoption. In this paper, we review some of the potential applications of GI in modern environmental science and discuss how reliability and associated (un)certainty in net benefits need to be addressed to realize the potential of this new approach.

Implementing green infrastructure for stormwater management and combined sewer overflow control

Implementing green infrastructure for stormwater management and combined sewer overflow control

Optimization of in-pipe storage capacity use in urban drainage systems with improved DP considering the time lag of flow routing

The active control of urban drainage systems (UDSs) is playing an increasingly important role in the world threatened by urban flooding and associated disasters caused by insufficient drainage capacity. However, little research has recognized the importance of the optimal use of in-pipe storage space. To address this issue, the use of the in-pipe storage capacity was optimized in this study. A novel approach, that is, dynamic programming with successive approximation considering the time lag of flow routing (DPSA-TL), was developed to determine the control policies, in addition to the commonly used passive, rule-based control (RBC), and evolutionary algorithm (EA) strategies. A real-life urban catchment considering flooding control and combined sewer overflow (CSO) reduction was used as the case study. First of all, the potential benefit of maximizing the use of in-pipe storage space was tested using the four control strategies in three storm events, including a 3-year, 2-hour design (46.5 mm), a 5-year, 2-hour design (56.0 mm) and a 7-h historical (152.5 mm) storm events. Results indicate that DPSA-TL performed best in all cases. Without compromising the goal of flooding control, it provided 16.5%, 12.6%, and 3.0% reductions in CSO volume for the three storm events when compared with the passive strategy. Due to the limited capacity of in-pipe storage, the relative improvement diminished as the total rainfall depth increased. Then, control strategies were further applicated to the real-time operation. DPSA-TL was found to be the best alternative for CSO control, with the CSO volume reduced by 14.7%, 11.4%, and 2.5% in the three storm events, respectively. The findings suggest that the performance of UDS can be significantly improved by optimizing the use of in-pipe storage capacity, and the proposed method is effective in the offline optimization and real-time control of UDSs.

Optimal Design of Combined Sewer Overflows Interception Facilities Based on the NSGA-III Algorithm

The interception facility is an important and frequently used measure for combined sewer overflow (CSO) control in city-scale drainage systems. The location and capacity of these facilities affects the pollution control efficiency and construction cost. Optimal design of these facilities is always an active research area in environmental engineering, and among candidate optimization methods, the simulation-optimization method is the most attractive method. However, time-consuming simulations of complex drainage system models (e.g., SWMM) make the simulation-optimization approach impractical. This paper proposes a new simulation-optimization method with new features of multithreading individual evaluation and fast data exchange by recoding SWMM with object-oriented programming. These new features extremely accelerate the optimization process. The non-dominated sorting genetic algorithm-III (NSGA-III) is selected as the optimization framework for better performance in dealing with multi-objective optimization. The proposed method is used in the optimal design of a terminal CSO interception facility in Wuhan, China. Compared with empirically designed schemes, the optimized schemes can achieve better pollution control efficiency with less construction cost. Additionally, the time consumption of the optimization process is compressed from days to hours, making the proposed method practical.

A Detention Reservoir Reduced Combined Sewer Overflows and Bathing Water Contamination Due to Intense Rainfall

Combined sewer overflows (CSOs) close to water bodies are a cause of grave environmental concern. In the past few decades, major storm events have become increasingly common in some regions, and the meteorological scenarios predict a further increase in their frequency. Consequently, CSO control and treatment according to best practices, the adoption of innovative treatment solutions and careful sewer system management are urgently needed. A growing number of publications has been addressing the quality, quantity and types of available water management and treatment options. In this study, we describe the construction of an innovative detention reservoir along the Arzilla River (Fano, Italy) whose function is to store diluted CSO wastewater exceeding the capacity of a combined drain system. River water sampling and testing for microbial contamination downstream of the tank after a heavy rain event found a considerable reduction of fecal coliform concentrations, which would have compounded the impact of stormwater on the bathing site. These preliminary results suggest that the detention tank exerted beneficial environmental effects on bathing water by lowering the microbial load.

Optimization of Real‐Time Control With Green and Gray Infrastructure Design for a Cost‐Effective Mitigation of Combined Sewer Overflows

AbstractAn innovative optimization‐simulation framework is applied to a case study of the Province of Quebec, Canada, to optimize the spatial distribution of green infrastructure (GI), the capacity and location of gray infrastructure, and the parameters specific to real‐time control (RTC) operating rules of a sewer system for reducing combined sewer overflows (CSOs) frequency and volume. GI, gray infrastructure, and RTC are applied either individually or in integration through eight optimization scenarios which are simulated over a nine‐year period of historical rainfall data. Among all scenarios, spatial optimization of GI with RTC leads to maximal CSO volume reduction (98%) and is the most cost‐effective option analyzed (70$/m3 of seasonal average CSO reduction compared to 140$/m3 for the scenario involving gray infrastructure alone). However, it requires a high GI implementation level and the CSO frequency under this scenario is sensitive to varying GI design parameters. The findings suggest that the best alternative for CSO control is the integration of the optimization of green and gray infrastructures with RTC as it still provides high CSO volume reduction (95%) and remains a cost‐effective solution (90$/m3 of CSO reduction), while providing robustness under cost and design uncertainties.

Assessing the system performance of an evolving and integrated urban drainage system to control combined sewer overflows using a multiple-layer based coupled modeling approach

Many earlier developed and densely populated metropolises served by combined sewer systems have been implementing additional stormwater control measures to reduce the impacts of combined sewer overflows (CSO) driven by severe rainstorms. These mitigation measures call for robust modeling strategies to assess the overall performance before development and to predict vulnerabilities, and examine the underlying mechanisms causing them during or after development in a systematic manner. This paper first introduces a hydrologic and hydraulic coupled modeling program-MetroFlow that interacts and integrates multiple models built via diverse methods and tools. A typical integrated urban drainage system (IUDS) in the greater land area of Chicago in the US was investigated as a testbed. In addition, a large-scale Tunnel and Reservoir Plan in construction was evaluated as a mitigation measure to alleviate CSO risks. In this effort, three models were coupled into MetroFlow to account for the best available data of the multiple structural layers involved in this evolving system. Hydrological responses were analyzed under existing and projected system conveyance and storage capacities considering the full spectrum of rainfall conditions employing synthetic storms and real-time simulations of four historical extreme events to account for the anomalies exemplified by individual natural events. The overall effectiveness and resilience of the system were further assessed by the evaluation of normalized performance indicators (PIs) quantifying the reduction of total volume, duration and spread of CSOs. The cluster of simulation results collectively suggests that: (a) the proposed structural layer-based modeling approach is applicable in reproducing distributed rainstorm-driven CSOs for an evolving IUDS based on emerging digital-physical data; (b) CSO characteristics are rather multivariate functions of the rainfall tempo-spatial characteristics as well as the hydraulic performance of the collection system and (c) consistent improvement of CSO abatement was predicted with increasing conveyance and storage provided by the deep tunnel system. The projected system is predicted to capture 24-h storms of return period up to 10 years. MetroFlow’s framework may also promote collaborative modeling, in which community-scale models can be utilized and merged to support extended-scale water and wastewater infrastructure planning, design and management.

Implementing green infrastructure for stormwater management and combined sewer overflow control

Implementing green infrastructure for stormwater management and combined sewer overflow control

Combined sewer overflows: A critical review on best practice and innovative solutions to mitigate impacts on environment and human health

Combined sewer overflows (CSOs) are of major environmental concern for impacted surface waterbodies. In the last decades, major storm events have become increasingly regular in some areas, and meteorological scenarios predict a further rise in their frequency. Consequently, control and treatment of CSOs with respect to best practice examples, innovative treatment solutions, and management of sewer systems are an inevitable necessity. As a result, the number of publications concerning quality, quantity, and type of treatments has recently increased. This review therefore aims to provide a critical overview on the effects, control, and treatment of CSOs in terms of impact on the environment and public health, strict measures addressed by regulations, and the various treatment alternatives including natural and compact treatments. Drawing together the previous studies, an innovative treatment and control guideline are also proposed for the better management practices.

Turbidity in Combined Sewer Sewage: An Identification of Stormwater Detention Tanks.

Combined sewer overflow remains a major threat to surface water quality. A stormwater detention tank is an effective facility to control combined sewer overflow. In this study, a new method for the selective collection of combined sewer sewage during wet weather based on real-time turbidity control is established to reduce the load of pollutants entering a river using a stormwater detention tank with a limited volume. There was a good correlation found between turbidity and the concentrations of total suspended solids (TSS) (R2 = 0.864, p < 0.05), total phosphorus (TP) (R2 = 0.661, p < 0.01), and chemical oxygen demand (COD) (R2 = 0.619, p < 0.01). This study shows that turbidity can be used to indicate the concentration of TSS, TP, and COD in the sewage of the combined sewer systems in wet weather. Based on the adopted first flush detection approach, total nitrogen (TN) and TP showed the first flush effect, whereas the first flush effect of TSS and COD was not obvious. The results show that it is impossible to effectively control combined sewer overflow by only treating the initial rainwater.

Research on Design Method Optimization of Combined Sewer Overflow Detention Tank

Detention tank is widely used in urban waterlogging control, rainwater runoff pollution control, combined sewer overflow (CSO) control and rainwater utilization, etc. For detention tanks in different types or functions, their design methods, construction forms and ways of operation and management are very different. In recent years, many cities in China have begun to apply or plan to build CSO detention tanks to control the CSO pollution, but there are still many problems and puzzles in designing the tank. This paper summarizes and evaluates the common design methods of CSO detention tank at China and abroad. Based on the physical projects in which the author has participated in, the scale and control effect of the CSO detention tank are studied by model simulation, taking a combined sewer system catchment in the Beijing old city as examples. Based on the analysis, the long-term simulation method is recommended for designing of CSO detention tanks in China, and a design criteria suitable for a larger range can be determined through research and analysis of a typical and representative area if there is no way to research every area. At last, several key factors that affect greatly the scale and pollutant control efficiency of CSO detention tank are summarized and analyzed, including objectives of CSO control, rainfall characteristics, CSO pollutants transport law, interception multiple, capacity of wastewater treatment plant, space conditions and investment, layout of detention tank and operation mode of detention tank. Hope to provide reference for the scientific designing of CSO detention tank and realize cost-effectively controlling CSO in cities of China.

MULTI-OBJECTIVE OPTIMIZATION OF COMBINEDSEWER OVERFLOW CONTROL SYSTEM

How to reduce the construction cost while increasing the reduction rate of combined sewer overflow (CSO) control system has been widely concerned by the planning & decision makers. In this study, the multi-objective optimization model of CSO system construction cost and CSO reduction rate was established by using Matlab, Python and SWMM. A series of Pareto solution sets were obtained using non-dominant sorting genetic algorithm (NSGA-Ⅱ), so different options were put forward to planning decision makers. For the 3-year storm events, the optimization results showed that the proportion of the construction cost was highest for constructing source low impact development (LID) facilities, followed by constructing the detention tanks and retrofitting the midway pipes. Among the three types of facilities, the contribution of the CSO reduction rate of the detention tanks was the highest, followed by the source LID facilities and the midway pipes.

Combining Hydrologic Analysis and Life Cycle Assessment Approaches to Evaluate Sustainability of Water Infrastructure: Uncertainty Analysis

The goal of this research is identifying major sources of uncertainty of an environmentally-sustainable urban drainage infrastructure design, based on hydrologic analysis and life cycle assessment (LCA). The uncertainty analysis intends to characterize and compare relative roles of unreliability, incompleteness, technological difference, and spatial and temporal variation in life cycle impact assessment (LCIA) data, as well as natural variability in hydrologic data. Uncertainties are analyzed using a robust Monte Carlo simulation approach, performed by High Throughput Computing (HTC) and interpreted by Morse-Scale regression models. The uncertainty analysis platform is applied to a watershed-scale LCA of rainwater harvesting systems (RWH) to control combined sewer overflows (CSOs). To consider the watershed-scale implications, RWH is simulated to serve for both the water supply and CSO control in an urban watershed in Toledo, Ohio, USA. Results suggest that, among the studied parameters, rainfall depth (as a hydrologic parameter) caused more than 86% of the uncertainty, while only 7% of the uncertainty was caused by LCIA parameters. Such an emphasis on the necessity of robust hydrologic data and associated analyses increase the reliability of LCA-based urban water infrastructure design. In addition, results suggest that such a topology-inspired model is capable of rendering an optimal RWH system capacity as a function of annual rainfall depth. Specifically, if the system could capture 1/40th of annual rainfall depth in each event from rooftops, the RWH system would be optimal and, thus, lead to minimized life cycle impacts in terms of global warming potential (GWP) and aquatic eco-toxicity (ETW). This capture depth would be around 2.1 cm for Toledo (given an 85 cm/year rainfall and 200 m2 typical roof area), which could be achieved through an RWH system with 4.25 m3 capacity per household, assuming a uniform plan for the entire studied watershed. Capacities smaller than this suggested optimal value would likely result in loss of RWH potable water treatment savings and CSO control benefits, while capacities larger than the optimal would likely incur an excessive wastewater treatment burden and construction phase impacts of RWH systems.

Effectiveness Analysis of Systematic Combined Sewer Overflow Control Schemes in the Sponge City Pilot Area of Beijing.

Combined sewer overflow (CSO) pollution poses a serious threat to the urban water environment and is more severe in old urban areas. This research uses the old urban area in the sponge city pilot area in Tongzhou District, Beijing, as the study area. The United States Environmental Protection Agency (USEPA) storm water management model (SWMM) was used to establish the hydrologic and hydraulic model of this area. The model parameters were calibrated and validated based on the measured rainfall and runoff data. The results show that the Nash-Sutcliffe efficiency coefficient for calibration and validation is more than 0.74. Thirty-two sets of systematic CSO control schemes are formulated, which include the "gray (includes the pipes, pumps, ditches, and detention ponds engineered by people to manage stormwater) strategy" and "gray-green strategies", and the regularity of CSO control for "low impact development (LID) facilities at the source", "intercepting sewer pipes at the midway", and "storage tank at the end", are quantitatively analyzed. The results show that the LID facility has an average annual reduction rate of 22% for the CSO frequency and 35% to 49% for the CSO volume. The retrofitting of intercepting sewer pipes has an average annual reduction rate of 11% for the CSO frequency and 4% to 15% for the CSO volume, and the storage tank has an average annual reduction rate from 3% to 36% for the CSO volume; furthermore, the reduction rate decreases with the increase in the CSO volume reduction rate by LID facilities. When the CSO control target is stricter, the control effect of the "end" segment is more obvious, but the control efficiency is lower. By studying the variability of the storage tank volume under different control targets, it can be concluded that it is reasonable to set the CSO control target because the number of overflow events does not exceed four times per year for the study area.

Selection of rainfall information as input data for the design of combined sewer overflow solutions

Combined sewer overflows (CSOs) cause environmental problems and health risks, but poor guidance exists on the use of rainfall data for sizing optimal CSO control solutions. This study first reviews available types of rainfall information as input for CSO modelling and, secondly, assesses the impacts of three rainfall data selection methods (continuous simulation, historical rainstorms selected based on rainfall depth or maximum intensity and IDF-derived storms) on the estimation of CSO volume thresholds to control in order to reach specific seasonal CSO frequency targets. The methodology involves hydrological/hydraulic modelling of an urban catchment in the Province of Québec (Canada). Continuous simulation provides the most accurate volume estimations and shows high sensitivity to the number of simulated years. Alternatively, when historical events extracted from rainfall data separated by a minimum inter-event time (MIT) criterion are selected based on their total rainstorm depth, the CSO volumes are underestimated significantly; whereas an analysis based on rainstorm maximum intensities over durations similar to the time of concentration provides more conservative volumes. Finally, synthetic storms constructed from multiple points of an IDF curve tend to underestimate slightly the CSO volumes, but provide acceptable results compared to single point derived storms. It was found that the overflow structures local characteristics had a marginal influence on results obtained from continuous simulation compared to event-based simulation. The use of design rainfall events should thus be restricted to preliminary assessment of CSO volume thresholds, and the final volume estimation for solution sizing should be reviewed under continuous simulation. The innovative contribution lies in the improvement of modelling procedures for solutions design to achieve a maximum CSO frequency, such as specified by many regulating agencies.